Should be a Dand 60. There should be a tag bolted down with 2 of the cover bolts. If it's got a part number like 60-60-something it's a Dana 60. You can also measure the height of the cover. If the bulge is about 10-3/4" from top to bottom, it's a D60. If it's about 11-3/4" to 12" it's a D70.
99 2500 auto gasser... what rear end?
